Set the readonly attribute on the hidden textarea when the editor is …

…read-only

This prevents cut/paste from showing up in the context menu on Chrome
(but doesn't help on Firefox).

Closes #6418

src/input/TextareaInput.js

@@ -366,6 +366,7 @@ export default class TextareaInput {
   readOnlyChanged(val) {
     if (!val) this.reset()
     this.textarea.disabled = val == "nocursor"
+    this.textarea.readOnly = !!val
   }
 
   setUneditable() {}